1. Field of the Invention
The invention relates to methods for manufacturing solid electrolytic capacitors each including a solid electrolyte layer containing an electrically conductive polymer layer formed by electrolytic oxidative polymerization.
2. Description of the Related Art
With respect to a solid electrolytic capacitor having an anode element made of a valve metal, a dielectric layer formed on a surface of the anode element, and a solid electrolyte layer containing an electrically conductive polymer layer and formed on the dielectric layer, the chemical oxidative polymerization method and the electrolytic oxidative polymerization method have been conventionally known as a method of producing the electrically conductive polymer layer. The former chemical oxidative polymerization method refers to a method of producing an electrically conductive polymer by allowing an oxidizing agent to act on a monomer and chemically oxidative polymerizing the monomer, and the latter electrolytic oxidative polymerization method refers to a method of producing an electrically conductive polymer by passing an electric current through an electrolytic polymerization solution containing a monomer to electrolytic oxidative polymerize the monomer. Generally, the electrolytic oxidative polymerization method can control conditions for polymerization easier than the chemical oxidative polymerization method and can produce an electrically conductive polymer excellent in electrical conductivity, mechanical strength and homogeneity, while the electrolytic oxidative polymerization method needs to use a more complicated manufacturing apparatus.
In order to form an electrically conductive polymer layer for use in a solid electrolytic capacitor by electrolytic oxidative polymerization method, an electrically conductive precoat layer of manganese dioxide produced by thermal decomposition of manganese nitrate or an electrically conductive polymer produced by chemical oxidative polymerization is first formed on a dielectric layer, because the dielectric layer, which is formed on the surface of a valve metal constituting an anode element, is an insulating material. Then, for example, the electrically conductive precoat layer and a metal plate of stainless steel or the like are used as an anode and a cathode, respectively, and an electric current is allowed to flow between the anode and the cathode in an electrolytic polymerization solution containing a monomer and a dopant agent also serving as a supporting electrolyte, so that the monomer is subjected to electrolytic oxidative polymerization to form an electrically conductive polymer layer (see, for example, Japanese Examined Patent Publication No. H04-74853).
However, there is a problem in which even using a solid electrolyte layer containing an electrically conductive polymer layer formed by electrolytic oxidative polymerization as described above, leak current in the resulting solid electrolytic capacitor is not reduced as expected, and when the electrolytic oxidative polymerization using the electrolytic polymerization solution described above is performed on an industrial scale, the production rate of solid electrolytic capacitors with large leak current becomes high.